Why would you need a new set of speakers? To be honest, I would go down to the best car audio place and tell them you want an amp and if they could wire it for you. All it does is amplify what the cd player tells it, everything should work fine as long as your running the same resistance (ie. 2 ohms, 4 ohms). I would get a kicker amp or JL.
http://www.crutchfield.com/S-SJKHfpwg6G1/cgi-bin/ProdGroup.asp?g=120&avf=Y